This invention relates to methods of and circuits for suppressing radar output due to clutter while retaining desired output due to targets that are moving with velocities different than those of clutter. The invention is applicable to radars that operate on the doppler principle of detection and therefore it is applicable to cw doppler, pulse doppler and MTI (moving traget indication) radars.
The principles of the three aforementioned types of radars are given in Chaps. 3 and 4 of the book Introduction of Radar Systems, McGraw-Hill Book Company, 1980, by M. I. Skolnik. Basically, for the three radar types, radar echo signals containing doppler frequency components are received and doppler filtering (processing) is used for selectively enhancing signals due to targets relative to signals due to clutter. Then, ideally, at the radar output target signals are unsuppressed by the doppler processing and clutter signals are of negligible strength because of attenuation caused by the doppler processing.
As explained herein under DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS, a variety of circuits and methods are employed for accomplishing the doppler processing (filtering) used for radars that employ the doppler principle. Even so, if the unprocessed radar clutter echo is much larger than the echo of a desired target, the radar output due to clutter for a cw, pulse doppler, or MTI radar can be non-negligible and therefore troublesome.
The usual technique for rejecting clutter at the output of a doppler processor is to employ an amplitude threshold level below which signals are rejected. Unfortunately, the threshold rejects the weaker targets and passes the stronger clutter and therefore amplitude thresholding is satisfactory only if, at the output of the doppler processor, the important target signals are stronger than most of the clutter. To improve the efficiency of thresholding radar output, CFAR (constant false alarm ratio) circuitry is used. The CFAR operates by sampling clutter and predicting future clutter levels based on the sampled clutter and assumptions about the variability of clutter. Although CFAR performance is sometimes reasonably effective against uniform clutter, it is an inefficient suppressor of clutter from non-homogeneous terrain.
Generally, ground clutter is not homogeneous and often consists of isolated and very strong clutter areas interspersed with areas of negligible clutter. The invention will effectively reject this isolated clutter even though it is very strong, and consequently the invention will improve the detectability of moving targets in an environment containing sources of strong clutter.